Multi-antenna techniques are techniques for improving communication capacity, frequency use efficiency, power consumption, etc., by performing transmission and reception using a plurality of antennas in radio communication. Note that even if the number of antennas is one on either the transmitting side or the receiving side, an improvement in communication quality, etc., can be achieved according to the number of antennas on the other side.
As a term related to such multi-antenna techniques, there is MIMO (Multiple Input Multiple Output). MIMO, when used as a communications term, often refers to a communication system where both the transmitting side and the receiving side use a plurality of antennas, but may be used referring to general multi-antenna techniques (see, for example, Non-Patent Literature 1).
There are the following four advantages to be obtained by a multi-antenna signal processing algorithm:
(1) Spatial diversity;
(2) Coherent gain;
(3) Interference mitigation; and
(4) Spatial multiplexing.
The spatial diversity refers to a reduction in degradation of communication quality caused by the influence of multipaths, etc., by the use of spatially separated antennas.
The coherent gain refers to an increase in the ratio between received power and noise in a desired direction by assigning weights using propagation path information (changes in amplitude and phase) to signals at the respective antennas on the receiving side and the transmitting side.
In the interference mitigation, receive signals from the respective antennas are weighted to cancel out incoming signals (interference signals) other than a desired signal, and combined. Interference signals smaller in number by one than the number of receive antennas can be removed. If the propagation factor of incoming signals is unknown, then some kind of learning algorithm needs to be used.
The spatial multiplexing is a method of simultaneously establishing a plurality of communication channels by applying the interference mitigation. There are a method in which communication capacity is increased by a single user transmitting different signals from a plurality of antennas, and a method in which frequency use efficiency is increased by a plurality of users simultaneously performing communication. The latter method is called SDMA (Space Division Multiple Access).